Detailed descriptions of mobile communication networks and systems can be found in the literature, such as in particular in Technical Specifications published by standardization bodies such as for example 3GPP (3rd Generation Partnership Project).
In such systems, a mobile terminal (or User Equipment UE) has access via a Radio Access Network (RAN) to a Core Network (CN) providing mobile communication services.
Examples of systems, such as for example 3GPP systems, include: 2G-GSM, 3G-UMTS, 3GLTE-EPS (Evolved Packet System).
For example, an architecture for EPS is recalled in FIG. 1 taken from 3GPP TS 23.401. EPS Core Network comprises a PS (Packet-Switched) domain called EPC (Evolved Packet Core). EPC comprises entities such as in particular MME (Mobility Management Entity) in charge of control plane functions such as in particular mobility management and paging procedure for MT (Mobile Terminating) services towards UEs in idle mode. In EPS, CS (Circuit-Switched) services (such as for example voice services or other services such as SMS (Short Message Service)) can be delivered as IP-based services, via PS domain EPC providing IP connectivity to an operator's IP network (for example IMS network).
On the contrary, in systems such as GSM or UMTS (not specifically illustrated), CS services (such as for example voice services or other services such as SMS (Short Message Service)) are delivered via CS (Circuit-Switched) domain. CS domain comprises entities such as in particular MSC/VLR or MSC Server/VLR) in charge of control plane functions such as in particular mobility management and paging procedure for MT (Mobile Terminating) services towards UEs in idle mode.
New functionalities may be introduced to allow reuse of already deployed CS infrastructures co-existing with newly deployed EPS infrastructures.
An example of such new functionalities is the functionality called CSFB (CS Fallback) and/or SMS over SGs in EPS, specified in particular in 3GPP TS 23.272. EPS architecture for CSFB and/or SMS over SGs in EPS is recalled in FIG. 2 taken from 3GPP TS 23.272.
The CSFB and/or SMS over SGs in EPS functionality uses SGs interface mechanisms between MSC/VLR (or MSC Server/VLR) in CS domain and MME in EPC supporting such functionality. SGs interface is specified in particular in 3GPP TS 29.118.
SGs is the reference point between MME and MSC/VLR (or MSC Server/VLR). The SGs reference point is used for the mobility management and paging procedures between EPS and CS domain. The SGs reference point is also used for SMS delivery.